In these years, unnecessary radiations of an electromagnetic wave from an electronic device are becoming of a problem. In particular, in keeping pace with increase in the use of a high-frequency electromagnetic wave, damages or impediments, such as malfunctions of a device or adverse effects to the brain or human bodies by an electro-magnetic noise (interferences), are being presented as new environmental problems.
To deal with these problems of EMI (Electromagnetic Interferences), it has become necessary to sufficiently diminish or prevent those undesirable reciprocal effects between an electronic device and another device. This may be made possible by suppressing radiations of an unnecessary electromagnetic wave from an individual electronic device that might obstruct regular operations of another device, or by increasing the resistant power of such another device against an electromagnetic wave emanating from the individual electronic device.
The operating principle of an electromagnetic wave suppressor having the electromagnetic wave absorbing capability, referred to below simply as the electromagnetic wave suppressor, is that the major portion of the energy of the incident electromagnetic wave is to be converted into the thermal energy within the interior of the electromagnetic wave suppressor. Thus, with the electromagnetic wave suppressor, it is possible to diminish both the energy penetrated towards its front side and the energy reflected towards its rear side.
The loss that occurs in converting the electromagnetic wave into the thermal energy may be classified into electrically conductive loss, dielectric loss and magnetic loss. A quantity of conversion of the electromagnetic wave into the thermal energy may be presupposed based on these three sorts of loss. In this case, the energy of the electromagnetic wave absorbed by a unit volume P[W/m3] may be expressed, in terms of the electrical field E, a magnetic field H and the frequency f, by the following mathematical expression (1):
                    P        =                                            1              2                        ⁢            σ            ⁢                                                          E                                            2                                +                      π            ⁢                                                  ⁢            f            ⁢                                                  ⁢                          ɛ              ″                        ⁢                                                          E                                            2                                +                      π            ⁢                                                  ⁢            f            ⁢                                                  ⁢                          μ              ″                        ⁢                                                          H                                            2                                                          (        1        )            where    electrical conductivity: σ    complex dielectric constant: ∈=∈−j∈″    complex permeability: μ=μ′−jμ″
In the above equation (1), the first term, second term and the third term stand for the electrically conductive loss, dielectric loss and the magnetic loss, respectively, and both j∈″ and jμ″ stand for delay components of the absorption of the electromagnetic wave.
At present, the electromagnetic wave suppressor is mainly used for an electronic device. In particular, the electromagnetic wave suppressor is used as it is stuck on a printed circuit board, an FPC (Flexible printed Circuit) or on an upper surface of a package. There has so far been developed a large variety of sorts of the magnetic sheet, such as the sheet containing a carbonaceous material, to say nothing of the sheet composed of a mixture of resin with ferrite or magnetic metal powders.
This electromagnetic wave suppressor is used as an absorber that absorbs an electromagnetic wave radiated from an antenna source or as a high harmonic filter suppressing that a high harmonic noise component superposes on a signal from e.g. an antenna source.
As the electromagnetic wave suppression material that suppresses an electromagnetic wave in a high frequency range, a magnetic material was prevalently used. With the magnetic sheet, formed of this magnetic material, the magnetic permeability μt″ of the third term of the equation (1) is set so as to be higher in order to absorb and suppress the electromagnetic wave.
Thus, among existing magnetic sheets, there is such a sheet in which the mixed quantity of magnetic powders is increased to elevate an absorption characteristic. In this case, the specific gravity of the magnetic sheet is increased with increase in the quantity of the magnetic powders to provide only a hard magnetic sheet which may not be appropriately bonded to e.g. the flexible printed board. Moreover, since the magnetic powders are used for the magnetic sheet, the sheet is costly. Hence, a need has been felt for a more inexpensive electromagnetic wave suppression sheet.
Thus, with attention focused on the dielectric constant of a liquid material having polarity or an ion-containing liquid material, such as an electrolytic solution, an electromagnetic wave suppression material having a high electromagnetic wave absorption efficiency has been developed for use in a high frequency range. Moreover, such an electromagnetic wave suppression material has also been developed which, through the use of a crosslinking structure, is able to remain in stabilized state against a set of changes in temperature.
Patent Publication 1 shows an electromagnetic wave suppression device as typical of the electromagnetic wave suppressor. The electromagnetic wave suppression device is composed of an electromagnetic wave suppression material sealed by a film adapted to prevent transpiration of a water-resistant vapor. The electromagnetic wave suppression material has been gelled by adding acrylamide as a resin component, methylenebisacrylamide as a curing agent, and ammonium sulfide as an initiator, to an electrolyte solution, such as sodium chloride. This electromagnetic wave suppression sheet, in which the electrolyte solution is kept in a state of a gel, has a high dielectric loss, up to a frequency range of 10 GHz, and hence exhibits the effect of suppressing unnecessary radiations of an electromagnetic wave in the high frequency range. This effect is referred to below as an electromagnetic wave suppression effect.
Patent Publication 1: Japanese Patent Publication Laid-Open No. 2006-73991